This invention relates to fuel injection pumps for internal combustion engines, and more particularly to fuel injection pumps for internal combustion engines capable of precisely controlling the amount of fuel injected and the injection timing.
Fuel injection pumps are used for feeding fuel at high pressure into the interior of internal combustion engines such as diesel engines. In recent years, a demand has been created for relying on electronic control for effecting control of the amount of the fuel injected and the injection timing of a fuel injection pump. A device of this type is shown in U.S. Pat. No. 4,185,779 issued Jan. 29, 1980, for example.
In the fuel injection pump incorporating therein the control device of this type, control of the amount of the fuel injected and the injection timing are effected by feedback control utilizing the servomechanism. A device of this type is complex in construction and high in cost.
An application for a patent (Ser. No. 304,359) was filed by the present application in the United States of America on Sept. 22, 1981 for a fuel injection pump capable of readily controlling both the amount of the fuel injected and the injection timing. The fuel injection pump comprises a rotor located in a cylindrical housing, a free piston movably mounted in an axial bore in the rotor, two pressure chambers defined by an inner wall surface of the axial bore in the rotor and the free piston, and two solenoid valves for supplying fuel to the two pressure chambers. One of the two pressure chambers receives therein the fuel to be injected and the other pressure chamber receives therein the fuel for effecting control of the injection timing. The two pressure chambers are separated from each other by the movable free piston.
The fuel injection pump described hereinabove is capable of readily controlling the amount of the fuel injected and the injection timing by virture of the aforesaid construction.
However, it is difficult to precisely measure an amount of fuel injected and make the timing control range sufficiently wide under the entire engine operation from low speed to high speed. The fuel injection pump distributes measured fuel to each cylinder in one rotation of the rotor, therefore, it is necessary to measure fuel, feed the pump chamber with the measured fuel, then effect compression and delivery of the fuel during the angular advance of the rotor corresponding to one of engine cylinders which is 90.degree. in case of a four cylinder engine. In this case, in order to carry out sure measurement of fuel injected, it is necessary to have a long operation time of the solenoid valve within the period of the measurement and feed of the fuel.
On the other hand, in order to increase the control freedom of fuel injection timing, it is necessary to start the operation of the pump chamber earlier. The early start is effected by making the slope for compression of a cam ring pressing a plunger longer. By this reason, in the fuel injection pump, the angular advance per one cylinder is divided into exact halves, one of which is for the measurement and feed and the other, for the compression and delivery. In such control, the measurement and feed are carried out at the same time, so that the rotational speed of the engine influences them greatly. In this pump, fuel from the solenoid value is fed to the pump chamber through an effective opening formed by communication of a rotary port and a stationary port. Therefore, at a low speed of the engine, the fuel is fed when the effective opening is opened insufficiently, and at a high speed, the effective opening disappears during the measurement of the fuel so that part of the fuel cannot be fed to the pump chamber. Thus, sufficiently precise or sure measurement of the fuel injected is difficult. Further, the injection timing control is effected by feeding another pump chamber with fuel measured by the solenoid valve for the injection timing control, and adjusting the time the plunger starts to contact the cam ring. In this case, also, the fuel introduction into the pump chamber for the timing control is effected through an effective opening formed by communication of the rotary port and the stationary port. Therefore, though the injection timing control range is widened by the compression slope of the cam ring, the long compression slope requires a larger angular advance, accordingly, which influences on the measurement of fuel to be injected.
Thus, the abovementioned fuel injection pump is required to be capable of more precise or sure control of an amount of fuel to be injected and injection timing.